The standard method for measuring linear displacement during room temperature fracture toughness tests is a clip gage extensometer instrumented with strain gages. The displacement is measured to allow the energy input into the specimen to be calculated. Also, periodic partial unloadings are made during the test. The slope of the load versus displacement curve is used to calculate the length of the crack throughout the test. This technique is referred to as the unloading compliance method.
For testing at elevated temperatures, the strain gages used within a clip gage extensometer would be destroyed. One solution to the problem of high temperature fracture toughness tests is to use a capacitance gage which can operate inside the furnace. The purpose of the attachment is to convey the linear displacement information from the deforming test specimen to an electrical sensor located out of the elevated temperature region. The type of sensor used with the prior art attachments is frequently, though not always, a linear variable differential transformer (LVDT).